This invention relates generally to the field of optical devices, and more specifically to an apparatus and method for switching, modulation and dynamic control of light transmission using photonic crystals.
Photonic crystals are a class of novel materials that offer new opportunities for the control and manipulation of light. Essentially, a photonic crystal consists of a periodic lattice of dielectric materials. For example, a single slab of semiconductor that hosts a periodic array of air holes or oxide columns. The underlying concept of photonic crystals originated from seminal work by Eli Yablonovitch and Sajeev John in 1987. The basic idea was to engineer a dielectric superlattice so that it manipulates the properties of photons in essentially the same way that regular crystals affect the properties of electrons therein. Electrons in a regular crystal see a periodic array of atoms, the coherent scattering felt by the electrons in the crystal will prevent any electron from traveling in the crystal if the energy of the electron unfortunately falls into certain ranges. Each continuous energy range of such a property is called an energy gap. By the same token, a photonic band gap exists for photons in a photonic crystal in a continuous range of frequencies where light is forbidden to travel within the photonic crystal regardless of its direction of propagation. For a three-dimensional photonic crystal, such a bandgap renders the crystal an omni-directional mirror that reflects incident light in any direction. For a two-dimensional photonic crystal, such a bandgap causes the crystal to completely reflect in-plane incident light.
Photonic crystal waveguides are essentially one-dimensional defects created inside a photonic crystal, often by removing a row of “atoms.” As the photonic crystal “walls” around the line-defect of a photonic crystal waveguides can completely reflect light for certain frequency ranges, light is forced to travel along the line-defect. Photonic crystal waveguides are known to be a promising element for constructing ultra-compact integrated optical devices than conventional integrated optical devices based on index-guiding schemes.
Active interferometer devices based entirely on photonic crystal waveguides were previously proposed by Soljacic et al. (see “Photonic-crystal slow-light enhancement of nonlinear phase sensitivity,” by M Soljacic, S. G. Johnson, S. Fan, M. Ibanescu, E. Ippen, J. D. Joannopoulos, Journal of Optical Society of America B 19, pp. 2052-2059; September 2002). These authors proposed that the slow group velocities of light in photonic crystal waveguides can be utilized to dramatically increase the induced phase shift owing to small changes in the refractive index. This enabled them to propose a number of designs of two-dimensional photonic crystal based switches and modulators significantly smaller in size than conventional devices. A thermo-optic photonic crystal modulator was demonstrated by Vlasov et al. (see “Active control of slow light on a chip with photonic crystal waveguides,” by Y. A. Vlasov, M. O'Boyle, H. F. Hamann & S. J. McNab, Nature, vol. 438, pp. 65-69; Nov. 3, 2005).
In some previously proposed Mach-Zehnder designs, little information is given regarding the structure and the means of changing the refractive indices of the constituent materials of a photonic crystal. Particularly, for an electro-optical modulator it is important to have a proper electrical structure that facilitates the coupling of electrical signals with light. Generally, proper design of electrical structures can greatly save power and increase the switching or modulation speed of an electro-optic device. For example, a structure can better achieve these goals if it allows maximum overlap between the region where light field is strongest and the region where the electrically induced refractive index change is strongest. On the other hand, lowering optical loss in such a device may place a constraint on achieving these goals. These design concerns have not been adequately considered previously.
Generally, an electro-optical modulator operating at 1 gigahertz or above is desired in optical communication and optical interconnection applications. If a device design can only function at a relatively low speed, then thermo-optic effect may dominate over the electro-optic effect or other effects. In a wide range of applications, an electro-optic switch or modulator working at high speed is always preferred to a thermo-optic modulator working at a low speed. Although there exist numerous conventional electro-optic modulators that do not comprise photonic crystals, they generally have large sizes and high power consumption. Hence electro-optical modulators based on new approaches are needed. Generally, an ultra-compact modulator with low heat generation is always preferred for use in integrated optoelectronic circuits.